1. Field of the Invention
This invention relates to a torque sensor in which a inverse magnetostrictive effect of an amorphous magnetic alloy is utilized so that the absolute value and direction of a torque applied on a rotary shaft can be detected without making any contact with the rotary shaft.
2. Description of the Related Art
A torque sensor using a magnetostrictive amorphous magnetic alloy is known and has a structure as shown in FIG. 11. Referring to FIG. 11, a pair of elongate strips 2a and 2b of the magnetostrictive amorphous magnetic alloy are helically bonded to a rotary shaft 1 to make angles of +45.degree. and -45.degree. respectively with respect to the axial direction of the rotary shaft. At corresponding locations outside of the portions of the rotary shaft 1 where the strips 2a and 2b of the amorphous magnetic alloy are bonded, a pair of coils 3a and 3b are disposed in a relation coaxial with and spaced apart by a predetermined distance from the rotary shaft 1, respectively. When a torque is applied on the rotary shaft 1, a strain is produced in each of the strips 2a and 2b of the amorphous magnetic alloy, and the permeabilities of the respective strips 2a and 2b change due to the inverse magnetostrictive effect of the amorphous magnetic alloy thereby causing corresponding changes in the inductances of the respective coils 3a and 3b. The change in the permeability of the strip 2a of the amorphous magnetic alloy helically bonded to the rotary shaft 1 at the angle of +45.degree. with respect to the direction of the thrust differs from that of the strip 2b of the amorphous magnetic alloy helically bonded to the rotary shaft 1 at the angle of -45.degree. with respect to the direction of the thrust. Therefore, when an output representing the difference between the inductances of the coils 3a and 3b is detected by suitable electrical circuits, the magnitude and direction of the torque applied on the rotary shaft 1 can be detected.
In the prior art torque sensor having the structure shown in FIG. 11, the strips of the amorphous magnetic alloy are bonded to the shaft (JP-A No. 59-77326). However, it is very difficult to make the coefficient of thermal expansion of the amorphous magnetic alloy to completely coincide with that of the material of the shaft. Accordingly, when the torque sensor is placed to operate in an environment where there is a relatively great change in the ambient temperature, the strips of the amorphous magnetic alloy are subjected to internal tensile deformation or internal compressive deformation in the directions parallel to the surface of the shaft from the shaft depending on the conditions including the bonding temperature of the amorphous magnetic alloy, the difference between the coefficient of thermal expansion of the amorphous magnetic alloy and that of the material of the shaft and the operating temperature of the torque sensor. A change in the operating temperature of the torque sensor results in a corresponding change in the amount of strain imparted from the shaft to the strips of the amorphous magnetic alloy. Thus, the prior art torque sensor has been defective in that not only the sensitivity to an applied torque changes but also a hysteresis occurs in the sensor output when the operating temperature changes.